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CHM 343 Separations > Exam 2: Gas and Liquid Chromatography > Flashcards

Exam 2: Gas and Liquid Chromatography Flashcards

1
Q

What is gas chromatography?

A

gaseous analyte is transported through the column by a gaseous mobile phase.

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2
Q

A gaseous mobile phase is also referred to as _________ ____.

A

carrier gas

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3
Q

Describe the GC process.

A

1) a volatile liquid or gas sample is injected through a septum into the heated port
2) Sample evaporates and is pulled through the column with a carrier gas
3) the column is heated to provide sufficient vapor pressure to elute analytes
4) analytes separately flow through a heated detection for observation.

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4
Q

The mobile phase in GC is usually a (solid/liquid/gas).

A

gas

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5
Q

What is usually the stationary phase in GC?

A

nonvolatile liquid, but sometimes a solid.

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6
Q

What is the usually the analyte in GC?

A

gas or volatile liquid

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7
Q

Name the three different carrier gasses.

A
  1. He
  2. N2
  3. H2
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8
Q

What does the choice of carrier gas depend on?

A

the detector and the desired separation efficiency and speed

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9
Q

What are the advantages of using open tubular column rather than packed columns?

A
  • higher resolution
  • shorter analysis time
  • greater sensitivity
  • lower sample capacity
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10
Q

Name the three types of open tubular columns.

A
  1. Wall-coated open tubular column (WCOT)
  2. Support coated open tubular column (SCOT)
  3. Porous-layer open tubular column (PLOT)
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11
Q

Describe a wall-coated open tubular column (WCOT)

A

A 0.1 -5 µm film of liquid stationary phase on the inside wall of the column

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12
Q

Describe a support-coated open tubular column

(SCOT)

A

solid particles coated with liquid stationary phase and attached to the inner wall.

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13
Q

Describe a porous-layer open tubular column (PLOT)

A

solid particles are the active stationary phase on inside wall of column

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14
Q

(narrow/wider) columns provide higher resolution than (narrow/wider) columns.

A

narrower; wider

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15
Q

what are the disadvantages of a narrower column?

A

require higher operating pressure and have less sample capacity.

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16
Q

What are the two types of columns?

A

1) Open Tubular Column
2) Packed Column

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17
Q

What is the range of a open tubular columns inner diameter, length, and thickness of stationary phase?

A

diameter: 0.10 to 0.53 mm
length: 10-100 mm
thickness: 0.1 -5 µm thick

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18
Q

Diameters greater than ____ mm tend to overload the vacuum system of a mass spectrometer.

A

0.32

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19
Q

what does increasing the thickness of the stationary phase of an open tubular column do at a constant linear velocity.

A

increases the retention time and sample capacity and increases resolution of early eluting peaks with a retention factor of k ≤ 5

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20
Q

Thick films of staionary phase can _______ tailing.

A

reduce

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21
Q

thick films of staionary phase can __________ bleed (decomposition and evaporation)

A

increase

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22
Q

List the three different types of open tubular columns in order of increasing surface area.

A

WCOT < SCOT < PLOT

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23
Q

List the three different types of open tubular columns in order of greater separation efficiency.

A

PLOT < SCOT < WCOT

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24
Q

For an Open tubular column, Rs increases with a ________ column

A

longer

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25
Q

Describe Packed Columns.

A

Solid fine porous particles coated with non-volatile liquid stationary phase.

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26
Q

How do Packed columns compare to Open tubular columns (OPTC)

A
  • Packed columns are shorter in length and wider in diameter
  • Has greater sample capacity
  • Give borader peaks
  • have less separation efficiency
  • longer retention times
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27
Q

Packed columns are usually used for what type of separations

A

Preparative

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28
Q

What is the range of diameter and length for a packed column?

A

diameter: 3-6 mm
length: 1-5 m

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29
Q

In a packed column, unifrom particle size (increases/decreases) the multiple path term in the van Deemter equation.

A

decreases

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30
Q

The Kovats retention index for Linear alkanes are equal to _____ times the number of carbon atoms.

A

100

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31
Q

For gas chromatography, _____________ temperature will decrease retention times of late-eluting components.

A

increasing

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32
Q

For gas chromatography, _____________ temperature will increase analyte vapor pressure

A
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33
Q

For gas chromatography, increasing temperature will _________ peaks for late-eluting components.

A

sharpen

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34
Q

What are the four signs of column degradation?

A
  1. Increased baseline noise at low temperature
  2. Peak boradening
  3. Tailing
  4. Retention time changes
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35
Q

What is column “bleeding”?

A

decomposition of stationary phase at high temperatures

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36
Q

For gas chromatography, Increasing inlet pressure (increases/decreases) the flow of the mobile phase and (increases/decreases) retention time.

A

increases; decreases

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37
Q

What does the low end of temperature limit mean for the column?

A

column can be kept for a long time if used at this temprature

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38
Q

What does the high end of temperature limit mean for the column?

A

column should only be exposed for a few mintues, short lifetime if used at this temperature. Column bleeding more likely to occur.

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39
Q

For analytes that are unstable at high temperatures, what elution technique should be used?

A

Pressure Programming

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40
Q

Name the two types of elution techniques.

A
  • Temperature Programming
  • Pressure Programming
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41
Q

Kovats retention index, I, for any given solute can be calculated from a chromatogram of a mixture of that solute with at least two _______ alkanes having retention times that bracket that of the solute.

A

normal

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42
Q

Analytes always elute in the (increasing/decreasing) order of Kovats index.

A

increasing

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43
Q

What is the most common carrier gas used with most detectors?

A

He

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44
Q

List the carrier gases in order of increasing optimal flow rate.

A

N2 < He < H2

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45
Q

Why does H2 and He give better resolution (smaller plate height) than N2 at high flow rates?

A

becuase solutes diffuse more rapidly which decreases mass transfer (Cux)

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46
Q

List the carrier gases in order of decreasing diffusion coeffcients.

**rentention factor, k, is the only variable that effects the rate of mass transfer, therefore is the diffusion coeff.**

A

H2 > He > N2

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47
Q

What is a guard column used for in gas chromtography?

A

accumulates nonvolatile substances that would contaminate the chromatorgraphy column

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48
Q

Label and define compenents of the air sandwhich Injection

word bank:

(Sample, solvent, 1st air, 2nd air, 3rd air)

A

1st air (far left): prevents the sample from evaporating

Sample: Analyte being injected

2nd air: prevents the sample and the solvent from mixing.

Solvent: used to wash the sample off.

3rd air: used to wash the solvent off.

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49
Q

Name the three different injection types used into open tubular columns.

A
  1. Split Injection
  2. Splitless Injection
  3. On- column Injection
50
Q

When is it preferred to use the method of Spilt injection over the others?

A
  • when analytes of interest constitute >0.1 % of the sample
  • Small sample size volume ( ≤1 µL)
51
Q

About how fast should samples be injected in a split injection?

A

< 1 s

52
Q

Describe the process a sample goes through within the instrument when injected by split injection.

A
  • sample is injected rapidly through the septum into the evaporation zone, which is kept at high temperature to promote fast evaporation. Septum purge also occurs to remove excess sample vapor.
  • carrier gas sweeps sample to the mixing chamber where vaporization and mixing occur.
  • Sample then travels to the split vent where a small fraction of vapor enters the chromatography column and the rest goes to waste
53
Q

In split injection, what is the sample that does not reach the column reffered to as.

and what does it range from.

A

split ratio

50:1 to 600:1

54
Q

When is it preferred to use the method of Spiltless injection over the others?

A
  • analytes that are less than 0.01% of the sample
  • best for trace levels of high-boiling solutes in low-boiling solvents
55
Q

Describe the process a sample goes through within the instrument when injected by splitless injection.

A
  • large sample (~2µL) of dilute solution in a low-boiling point solvent is injected slowly (~2s) with split vent closed
  • 80 % of sample is used applied to the column, and little fractionation occurs.
56
Q

How does splitless injection differ from split injection.

A
  • spliless injection does not use a mixting chamber
  • Injection volume is larger (~2µL)
  • injection time is longer (~2 sec)
  • amount of sample applied to column is larger (80% of sample)
57
Q

For splitless injection technique, solvent trapping, temperature at the column head is set ______0 C (above/below) the boilng point of the _______.

A

40; below; solvent

58
Q

Explain how solvent trapping works in splitless injection.

A
  • solvent condenses at the beginning of the column, then solutes catch up with the condensed solvent and become trapped in the solvent band.
  • Chromatography is initiated by raising the column temperature to evaporate and vaporize the solutes and solvent

**Note: without solvent trapping, there would not be as sharp chromotographic peaks.**

59
Q

For splitless injection technique, cold trapping, temperature at the column head is set ______0 C (above/below) the boilng point of the _______.

A

150 ; below; analyte

60
Q

Explain how cold trapping works in splitless injection.

A
  • Solvent and low-boiling components are eluted rapidly, but high boilng analytes remain in a narrow band at the beginning of the column.
  • Chromatography is initiated by rasing the temoerature and allowing analyte to evaporate.
61
Q

When is it preferred to use the method of On-Column Injection over the others?

A
  • best for thermally unstable solutes and high-boiling solvents;
  • best for quantitative analysis
62
Q

Describe the process a sample goes through within the instrument when injected by on-column injection

A

soultion is injected directly into the column, without going through a hot injector.

63
Q

What characteristics does on-column injection have that differs from the other two types of injections?

A
  • No mixing chamber
  • direct injection into the column
  • very little loss of solute
64
Q

When using the method of on-column injection, the initial oven temperature is kept (low/high) in order to trap all solutes in narrow band.

A

low

65
Q

List the detectors used for gas chromatography?

A
  • Thermal Conductivity Detector (TCD)
  • Flame Ionization Dectector (FID)
  • Electron Capture Detector (ECD)
  • Mass Spectrometer
66
Q

Which carrier gas gives the lowest detection limit with thermal conductivity detectors?

A

H2

He

67
Q

What does a Thermal Conductivity Dectector do?

A

measures the ability of a substance to transport heat from a hot region to a cold region by measuring the conductance in relation to temperature.

68
Q

Any _______ that is mixed with the carrier gas (highers/lowers) the conductivity of the gas stream, (increasing/decreasing) the temperature of the detector lock.

A

analyte; lowers; increasing

69
Q

With thermal conductivity detectors, as conductivity goes down, resistance goes ________

A

up

70
Q

What is the relationship between the sensitivity of the thermal conductivity detector and the flow rate.

A

sensitivity of thermal conductivity detector is inversely proportional to the flow rate.

71
Q

What are the advantages of using a thermal conductivity detector?

(5 advantages)

A
  • Simple and Universal
  • Detects organic and inorganic compounds
  • Non-constructive
  • Allows detection of solutes after detection
  • Passes solutes to a second detector
72
Q

Whats are the drawbacks of using a thermal conductvity detector?

(2 drawbacks)

A
  • Responds to impurities, decomposed staionary phase, or air that leaks into the system
  • low sensitivity
73
Q

How does a Flame Ionization Detector work.

write the representative equation.

A

eluate is burned in a mixture of H2 and air. Carbon atoms from organic compunds produce ions in the flame.

CH + O —-> CHO+ + e-

74
Q

In a flame ionization detector, a __________ monitors the amount of ions from the reaction

A

collector

75
Q

What type of carbons cannot produce ions and electrons in the flame

A

carbonyl and carboxyl carbons

76
Q

What is the most widley used detector in GC?

A

Flame Ionization Detector

77
Q

List the advantages of using a Flame Ionization Detector.

A
  • Mass Sensitive
  • Independent of flow rate
  • 100~1000 fold better than TCD
  • Most widely used detector in GC
78
Q

List the Drawbacks of using a Flame Ionization Detector.

A
  • Insensitive to non-hydrocarbons and non-combustible gases (H2O, CO2 , SO)
79
Q

How does a Electron Capture Detector (ECD) work?

A

Electrons travel between a 63Ni electrode and a collector electrode. Analytes with electron capturing ability pass through the cell and decrease the flow of electrons.

80
Q

In an electron capture detector, what is the relationship between the change in current and analyte concentration?

A

the change in current is proportional to the analyte concentration.

81
Q

List the advantages of an electron capture detector?

A
  • Sensitive to halogens, nitriles, carboxyl, and nitro compounds
  • Non-destructive
82
Q

List the drawbacks of using an Electron Capture Detector.

A
  • A radioactive source is used
  • Analytes must be able to capture electrons
83
Q

List the other detectors

A
  • Nitrogen-Phosphorous Detector (NPD)
  • Flame photometric detector
  • Photoionization detector
  • Sulfur/Nitrogen chemiluminescence detector
84
Q

What does a Mass Spectrometer do?

A

converts eluting analytes into gas phase ions, forms a molecular ion and breaks molecular ions into fragments which are then eluted based on thier mass to charge ratio

85
Q

Which GC detector is best for detecting organic compounds?

A

Flame Ionization Detector

86
Q

Which detector is best for detecting compunds with electronegative groups?

A

Electron Capture Detector

87
Q

Which detector is best for detecting compounds containing N and/or P?

A

Nitrogen-Phosphorus Detector

88
Q

Define Sample Preparation

A

Transforming the sample into a form that is suitable for analysis.

89
Q

Name the four different techniques of sample preparation applicable to GC

**that were discussed in class**

A
  1. headspace analysis
  2. Purge and trap
  3. Thermal Desorption
  4. Solid-Phase MicroExtraction (SPME)
90
Q

Describe Headspace analysis

A
  • the sample phase is introduced into a vial and the vial is sealed.
  • components diffuse into the gas phase until headspace has reached a state of equilibrium.
  • The sample is taken from the headspace with a gas sample syringe
91
Q

Describe Purge and Trap

A

bubble purge He gas is mixed with a heated sample to evaporate analytes, and are captured on an adsorbant column.

**extracts 100% of analyte from sample**

92
Q

Describe Solid-Phase Microextraction

A

extracts compounds from liquids or air without using any solvent by being absorbed on a coated fiber

**only extracts a portion of analyte from sample**

93
Q

What five topics do you consider when choosing a proper method development for GC?

**in a specific order**

A
  1. Goals of analysis
  2. Sample Preparation
  3. Detector
  4. Column
  5. Injection
94
Q

In liquid chromatography, the efficiency of a packed column increases as the size of the stationary particles (increases/decreases).

A

decreases

95
Q

List the chromatographic process

A
  • Sample Injection
  • Sample Transportation
  • Column separation
  • Analyte detection
  • Analyte exiting
96
Q

Describe how a 6-port injection valve in the load position works.

A

Sample loop is filled with sample solution by applying pressure on it, excess sample is delivered to waste, and solvent is continously pumped through the column.

97
Q

How much sample can a sample loop hold?

A

2 to 1000 µL

98
Q

Describe how a 6-port injection valve in the Inject position works.

A

from the load position, the valve is rotated 600 clockwise and the solvent pushes the analyte solution in the sample loop into the column

99
Q

What is a guard column?

A

a short column that is placed in front of the main column and used to remove particle and contaminants from the solvent.

100
Q

What is the most common stationary phase used in LC?

A

silica

101
Q

Name the 5 types of HPLC techniques

A
  1. Partition Chromatography
  2. Adorption Chromatography
  3. Ion-Exchange Chromatography
  4. Size Exclusion Chromatography
  5. Affinity Chromatography
102
Q

normal-phase chromatography uses a (polar/nonpolar) stationary phase

A

polar

103
Q

In normal-phase chromatography, (most/least) polar component is eluted first.

A

least

104
Q

In normal phase chromatography, the (more/less) polar solvent has higher eluting strength

A

more

105
Q

In normal-phase chromatography, inreasing the polarity of the solvent (increases/decreases) the elution time.

A

decreases

106
Q

Reverse-phase chromatography uses a (polar/non-polar) stationary phase.

A

non-polar

107
Q

In Reverse-phase chromatography, the (most/least) polar component is eluted first.

A

most

108
Q

In Reverse-phase chromatography, increasing the polarity of the mobile phase (increases/decreases) the elution time.

A

increases

109
Q

What are the requirements of a solvent to act as a mobile phase in HPLC.

A
  • Must be HPLC grade
  • must prevent contamination and damages to columns.
110
Q

what are the two types of programmed elution techniques used for HPLC?

A
  • Isocratic Elution
  • Gradient Elution
111
Q

What is Isocratic Elution?

A

a HPLC elution technique performed with a single solvent or constant composition solvent mixture

112
Q

What is Gradient Elution?

A
  • One or more solvents
  • continously changing in solvent composition
  • Fast elution for late-eluting species
  • Re-equilibrate at the beginning of each one
113
Q

In HPLC, What are the different treatments that can be used on sample to ensure no contamination

A
  • Sonication
  • Filtration
  • Sparging
114
Q

What is sonication?

A

Removes bubbles from sample

115
Q

what is filtration?

A

Removes particles from sample

116
Q

What is Sparging?

A

Removes O2 by an introduction of an inert gas

(purging with gas)

117
Q

List optimization steps used to achieve optimal separation in HPLC.

A

1) Acetonitrile and water
2) Methanol and water
3) Tetrahydrofuran and water

118
Q

List the suggested order of steps to try separating two closely spaced peaks in HPLC.

A
  1. Change the solvent strength
  2. change the temperature
  3. change the pH (in small steps)
  4. use a different solvent
  5. use a different kind of stationary phase
119
Q

What is the desired Rs?

A

>1.5

120
Q

what range of k (retention factor) value is an adequate separation in a reasonable time?

A

0.5~20

121
Q

what is ion-exchange chromatography?

A

charged stationary phase attracts solute ions

122
Q
A

CHM 343 Separations (3 decks)

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